Steel used as a material for internal and external plates of automobiles such as doors, hoods, fenders, floors and the like is required to have not only high strength but also excellent formability, to ensure the safety of passengers from accidents and to improve fuel efficiency by reducing the weight of cars.
However, since an increase in the strength of steel sheets may cause a deterioration of formability, it may be very difficult to satisfy both of the above factors, strength and formability, at the same time. Particularly, in the case of components requiring relatively high formability, such as internal door panels, rear floor panels and the like, since defects in formability such as the occurrence of cracks in a process often occurs, the application of high strength steels to such components is limited.
As a known steel sheet having excellent strength and formability, having been developed to date, there is so-called interstitial free (IF) steel to simultaneously secure strength and formability, by adding strong carbonitride forming elements such as titanium (Ti) and/or niobium (Nb) or the like to remove a solid solution element such as carbon (C), nitrogen (N), sulfur (S) or the like, which is representatively disclosed in Patent Documents 1 to 4. However, the IF steel has an average plastic anisotropy coefficient (lankford value, r value) of 1.5 to 1.8, and is insufficient to substitute for components for which an existing deep-drawing-quality (DDC) soft cold rolled steel sheet is used.
(Patent Document 1) Japanese Patent Laid-Open Publication No. 1992-280943
(Patent Document 2) Japanese Patent Laid-Open Publication No. 1993-070836
(Patent Document 3) Japanese Patent Laid-Open Publication No. 1993-263184
(Patent Document 4) Japanese Patent Laid-Open Publication No. 1998-096051